This invention relates to the field of testing electrical circuits. In particular, this invention relates to an apparatus with removable beam probes for testing bound electrical circuits.
In recent years electrical circuits become drastically more complex and smaller at the same time. The numbers of contact pads for transmitting electrical signals from an integrated circuit is rising and the size of the pads is becoming smaller and smaller. Such integrated circuits are usually tested while still on a wafer, or in the bound state. For this purpose the contact pads are contacted by suitable probes which send a series of electronic test signals through the integrated circuit.
A number of prior art design concepts teach cantilever probes for laterally accessing the pads. Other prior art techniques teach to access the pads directly from above by using mechanically resilient, conductive beam probes.
Various design solutions for arranging and guiding those so called xe2x80x9cbuckling beam probesxe2x80x9d have been developed. For more information about buckling beams and probe devices employing them see, for instance U.S. Pat. Nos. 3,806,801; 4,506,215; 4,518,910; 4,686,464 and 4,901,013.
The prior art teaches to bind or attach such buckling beam probes in a mechanically rigid way on one end by soldering them into sockets. Unfortunately, the adhesive and thermal influences during the soldering process disturb the alignment of the beam probes. Therefore, an additional alignment process is required as a final step in the manufacturing of such beam probe assemblies. The thinner those buckling beams become, the more difficult this alignment process becomes.
Before an electrical circuit can be tested, it must be placed in a testing position on a stage. The stage applies an overdrive and lifts the circuit so that each buckling beam contacts a predetermined pad. In most cases the contacting tip of the buckling beam scrubs along the surface of the pad, thereby removing an insulating oxide layer which forms on top of the pad. Those oxidized layers are hard and cause the buckling beam""s tip to become abraded over many test cycles. This abrasion does not occur evenly on all pads, causing individual beam tips to abrade faster than others. In addition mechanical stress and irregularities during the testing cycles always cause individual buckling beams to lose their original manufactured shape.
Thus there exists a need for a probe apparatus capable of assembling the beam probes in a way, such that the abrasive processing of the probe tips for alignment and maintaining can be avoided. Additionally, it would be an advance over the prior art to provide a probe apparatus in which the beam probes can be individually replaced. Thus, damaged beam probes could be simply removed and new beam probes inserted in their place.
Accordingly, it is a primary object of the present invention to provide a probe apparatus equipped with beam probes which are self-supporting, resilient and removable. It is a further object of the invention to assembly the beam probes in such a way, that during the testing process the beam probes are kept in aligned position to assure the same contact characteristic for each individual beam probe with the designated pad. It is another object of the invention to assembly the beam probes in a way, that a predetermination of the spring characteristic is possible. It is an additional object of the invention to assembly beam probes with a minimal lateral deformation to achieve the highest possible beam probe density. Further objects and advantages will become apparent upon reading the detailed description.
The object and advantages of the invention are secured by a probe apparatus for contacting pads of an electrical circuit under test. The probe apparatus has a space transformer with a number of electrically conductive holes. The conductive holes can be plated with a conductive material such that they offer an electrically conductive inner surface. The conductive inner surfaces of the holes are connected via horizontal and vertical conductors to connectors on the surface of the space transformer. Those connectors are in electrical communications with the circuitry required to perform the test. The space between the connectors is sufficient for accommodating cables for transmission of other electrical signals. The horizontal and vertical conductors are part of conductor carrier plates, which are a stacked and bound to form a part of the space transformer.
Electrically conductive beam probes are inserted into the conductive holes. The beam probes have a probe tip for contacting the pads, a beam section and a probe neck with a mechanically resilient feature. The beam probes are inserted into the holes by their necks such that the resilient feature frictionally retains the neck. As a result, the beam probe is retained in the hole due to friction resistance.
The mechanically resilient feature retaining the beam probes in the holes can be an undulating section such as a section having a spiral form or a sections with undulations confined to one plane. Alternatively, the resilient feature can be a lateral deformation or a number of deformations which may contact the hole on two opposite sides and at a number of contact points.
Preferably, the holes terminate in a bottom surface and the probe neck of each beam probe rests against this bottom surface. This approach allows the user to achieve better planarity between the probe tips. In addition, the holes may have a rotational positioning feature, such that the neck is urged into a predetermined rotational orientation when inserted. In a first embodiment the probe neck comprises one, two or more lateral manufactured deformations exceeding the contour shape of the conductive hole in disassembled comparison. The lateral deformations can have an undulating shape, expanding in one or more planes or in spiral arrangement. A resilient lateral urging is imposed at a number of contact points on the probe when inserted into the conductive hole.
The probe apparatus can additionally have plates with guiding holes for receiving the beam sections of the beam probes. Also, one or more auxiliary plate can be provided. The holes in the plates can be offset from the holes in the space transformer. In one embodiment, the probe apparatus has a mechanism for laterally displacing the plate and changing the lateral offset. It should also be noted that the holes in the plate can be offset from the holes in the auxiliary plates.
The position and amount of resilient urging of the beam contact points in the conductive hole defines a rotational urging or torque on the beam probe, wherein the rotational urging from only two beam contact points is free of any deflective influence on the probe neck itself. This rotational urging is transmitted onto the beam section and opposed through the guiding holes of one or more of the guiding plates. Such rotational positioning feature may simply be provided by holes with oval or elliptical cross-sections.
In a second particular embodiment the lateral urging on the probe neck is imposed by a rotational urging of the probe neck exceeding the hole boundaries. The rotational urging is imposed by a predetermined bending of the beam section by offsetting the guiding holes of one or more guiding plates out of the alignment along the manufactured shape of the beam section.
The predetermined bending provides a desired spring characteristic over the length of the beam probe against impact of the pad moving towards the probe tip while the bound electrical circuit is placed into testing position.
In yet another embodiment utilizing a first plate with guiding holes the beam probes have necks with mechanically resilient sections. When the necks are inserted into the guiding holes the plate is moved by an adjustment mechanism which laterally displaces the plate. This introduces a lateral offset between the holes in the space transformer and the guiding holes causing the mechanically resilient section to be frictionally retained in the conductive hole. Thus, the beam probes are retained in the space transformer. Of course, the mechanically resilient section may have a mechanically resilient feature such as a lateral deformation or a section which is straight when no mechanical stress is applied.
The details of the invention are explained in the detailed description in reference to the attached drawing figures.